The long-term goal of this research is to understand the mechanisms eukaryotic cells use to regulate the assembly of cilia and flagella. The flagella of Chlamydomonas are maintained at a constant length, but mutations in four different genes: LF1, LF2, LF3 and LF4, cause the cells to lose control of assembly and grow flagella up to three times normal length. Two of these genes encode members of well-studied gene families: a MAP kinase and a kinase of the CDK family. Analysis of the four LF genes leads to the conclusion that LF4, a novel MAP kinase, acts to enforce flagellar length control by shortening the flagella. LF1, LF2 and LF3 appear to act together to regulate length, perhaps by regulating retrograde intraflagellar transport (IFT). During the next project period the specific aims of this project will address the following questions:l) What proteins regulate the MAP kinase enzyme encoded by the LF4 gene, and what proteins does LF4p phosphorylate? 2) How do the proteins in the putative LF1/LF2/LF3 cytoplasmic complex interact, and how does this complex regulate the LF2 protein kinase? 3) What are the protein substrates of the LF2 protein kinase and how does phosphorylation of these targets regulate flagellar length? 4) Do the long-flagella mutants show alterations in intraflagellar transport (IFT)? Cilia and flagella have long been known to play key roles in processes that involve the movement of fluids over surfaces or the movement of cells through fluids. More recently it has become clear that cilia and flagella have critical functions early in development. Defects in the assembly of cilia and flagella have been connected to polycystic kidney disease retinal degeneration, situs inversus, and other problems in mammalian development. Understanding the regulation of flagellar assembly in Chlamydomonas provides a powerful model for understanding this assembly in mammalian systems.